1. Field of the Invention
The present invention relates to an information storage device such as a magnetic disk device, and particularly to an information storage device capable of automatically adjusting a write current on the basis of a measured value of VMM (Viterbi Metric Margin), a write current adjustment method for the information storage device, and a write control circuit. Note that VMM is sometimes called CSM (Channel Statistics Measurement). However, the term of VMM is selected in the present specification.
2. Description of the Related Art
Setting values such as a write current value and an overshoot value (abbreviated as an O/S) in a magnetic disk recording/reproducing device which forms part of a magnetic disk device vary depending on the writing ability of a head, the way or condition of writing into a medium, and the like. Therefore, in usual cases, a margin is measured and the setting values are determined so as to optimize the margin. This method aims mainly at optimization against differences in characteristics between media and in writing performance between heads. Importance is hence given to writability. Known reference technology as prior art is, for example, Patent Document 1 (Jpn. Pat. Appln. Laid-Open Publication No. 2005-93054.
However, recording density and track density have been raised to be higher in recent years. As importance has been given to writing ability of a head, a problem has arisen in that spreading of a writing spot causes erasure on tracks adjacent to or in the periphery of a written track. In other words, there conspicuously appears a phenomenon that side erasure occurs more easily as the writing ability of a head or medium is raised higher. This side erasure causes the following problems, for example.
At first, as the first problem, if the write current is set with a VMM known as a conventional method used for optimizing the write current, an offset margin, and a TPI (Track Per Inch) margin, there appears a phenomenon that side erasure is caused particularly by a combination of a head and a medium which have high writability. The VMM is defined by a number in case where difference between metric values based on two paths decreases below a certain threshold value. The VMM is given as a total sum when reading 100 sectors (about 400,000 bits).
Side erasure is a phenomenon as follows. That is, in addition to a magnetic field from an write gap which has been originally existing, a magnetic flux leaks from a write magnetic pole to a portion other than the track on which data should originally be written, thereby to erase original data. Depending on the shapes of magnetic poles of the head, this phenomenon easily occurs.
FIG. 3 shows write/read area 1 with respect to a write core WC of a data head, an outer adjacent tracks 2 and an inner adjacent track 3 on both sides of the region 1, a gap write area 4, and areas 5 where side erasure occurs.
The second problem is signal quality immediately after starting writing. At an actual start of writing, writing performance is ordinarily worse than in a steady state. This is greatly influenced by an element that is near a write head and projects as a write current flows. When the write current starts flowing, the element near the write head floats up high due to the projection. The floating height of the element thereafter falls gradually.
When the write current comes into a steady state, i.e., while a write operation continues, the projection of the element is substantially in a steady state. With respect to floating during writing, normally, as the floating height from a medium becomes lower, the writing ability rises.
Considering a period immediately after writing, the projection of the element by the write current is not yet stable, and as a result, the floating height of the element is higher than in a steady state. Therefore, the writing ability is still low. This is usually called a low lead O/W. If writing is executed in this state, there is a case that the lead margin deteriorates extremely and the signal quality lowers. In this case, the write current and O/S value need to be increased, on the contrary to the direction of solution to the first problem. In actual, however, no optimum values have been determined in consideration of these problems.